A Re‐evaluation of the Plenus Cold Event, and the Links Between CO₂, Temperature, and Seawater Chemistry During OAE 2

Abstract

The greenhouse world of the mid-Cretaceous (~94 Ma) was punctuated by an episode of abrupt climatic upheaval: Oceanic Anoxic Event 2. High-resolution climate records reveal considerable changes in temperature, carbon cycling, and ocean chemistry during this climatic perturbation. In particular, an interval of cooling has been detected in the English Chalk on the basis of an invasive boreal fauna and bulk oxygen-isotope excursions registered during the early stages of Oceanic Anoxic Event 2-a phenomenon known as the Plenus Cold Event, which has tentatively been correlated with climatic shifts worldwide. Here we present new high-resolution neodymium-, carbon-, and oxygen-isotope data, as well as elemental chromium concentrations and cerium anomalies, from the English Chalk exposed at Dover, UK, which we evaluate in the context of >400 records from across the globe. A negative carbon-isotope excursion that correlates with the original "Plenus Cold Event" is consistently expressed worldwide, and CO 2 proxy records, where available, indicate a rise and subsequent fall in CO 2 over the Plenus interval. However, variability in the timing and expression of cooling at different sites suggests that, although sea-surface paleotemperatures may reflect a response to global CO 2 change, local processes likely played a dominant role at many sites. Variability in the timing and expression of changes in water mass character, and problems in determining the driver of observed proxy changes, suggest that no single simple mechanism can link the carbon cycle to oceanography during the Plenus interval and other factors including upwelling and circulation patterns were locally important. As such, it is proposed that the Plenus carbon-isotope event is a more reliable stratigraphic marker to identify the Plenus interval, rather than any climatic shifts that may have been overprinted by local effects.

New constraints on the age, geochemistry, and environmental impact of High Arctic Large Igneous Province magmatism: Tracing the extension of the Alpha Ridge onto Ellesmere Island, Canada

“…Numerous studies have suggested a linkage of one of the most intense OAEs, the late Cenomanian OAE2 (≈94 m.y. ), to either (or both) the HALIP and Caribbean LIP (e.g., Kuroda et al, 2007, Turgeon andCreaser, 2008;Adams et al, 2010;Tegner et al, 2011;Zheng et al, 2013;Löhr and Kennedy, 2014;Du Vivier et al, 2015;Schröder-Adams et al, 2019;O'Connor et al, 2020). Several studies have used geochemical proxies to link OAE2 to a purported submarine LIP eruption, including: isotopic records of Os (Turgeon and Creaser, 2008;Du Vivier et al, 2015;Schröder-Adams et al, 2019), S (Adams et al, 2010), Nd (Zheng et al, 2013), and Hg (Percival et al, 2018).…”

“…Several studies have used geochemical proxies to link OAE2 to a purported submarine LIP eruption, including: isotopic records of Os (Turgeon and Creaser, 2008;Du Vivier et al, 2015;Schröder-Adams et al, 2019), S (Adams et al, 2010), Nd (Zheng et al, 2013), and Hg (Percival et al, 2018). This has important implications for how climate and environmental impacts during OAE2 are modeled, as it implies deleterious volatiles of the LIP eruption are released directly into marine waters rather than into the atmosphere (e.g., O'Connor et al, 2020;Joo et al, 2020;Percival et al, 2018). However, it has been argued that the release and distribution of magmatic volatiles is greatly reduced in submarine environments compared to subaerial eruptions (e.g., Percival et al, 2018).…”

Cenomanian-Turonian sea-level transgression and OAE2 deposition in the Western Narmada Basin, India

“…The OAE2 has a 2 ‰ to 3 ‰ positive δ 13 C excursion, prolonged plateau and gradual decrease across the Cenomanian-Turonian boundary in shallow marine environments Gertsch et al, 2008Gertsch et al, , 2010El-Sabbagh et al, 2011;Jarvis et al, 2011;Bomou et al, 2013;Wendler, 2013;Kuhnt et al, 2017). Shallow neritic environments reveal this characteristic δ 13 C excursion dependent on the continuity of the sedimentation record (Gertsch et al, 2008(Gertsch et al, , 2010O'Connor et al, 2020). We illustrate the OAE2 δ 13 C excursion for Pueblo, Colorado, and the Sinai Wadi El Ghaib sections compared with the western Narmada Basin of India (Fig.…”

“…Presence of the global OAE2 δ 13 C excursion in the open ocean and shallow nearshore sequences is an excellent correlation tool, provided sedimentation is relatively complete. The δ 13 C excursion began with a gradual rise to the maximum excursion coupled with the short Plenus cold event, also identified as the Plenus Carbon Isotope Excursion (CIE) (O'Connor et al, 2020). This event merged into a broad fluctuating δ 13 C plateau, followed by the gradual decrease to normal δ 13 C values in the early Turonian (Gertsch et al, 2008(Gertsch et al, , 2010Jarvis et al, 2011;O'Connor et al, 2020).…”

“…Journal Pre-proof The W. archaeocretacea zone is subdivided into three subzones Keller and Pardo, 2004), two of which span the Plenus Cold Event and maximum δ 13 C excursion ( Fig. 6) (Keller and Pardo, 2004;O'Connor et al, 2020). Subzone Globigerinelloides bentonensis defines the interval between the R. cushmani extinction at the base and the G. bentonensis extinction at the top, but also coincides with the first appearances (FAs) of D. imbricata at the base and D. hagni at the top.…”